There has been known a technique in which disposes an NOx storage reduction catalyst (hereinafter referred to as an NSR catalyst) is disposed in an exhaust passage of an internal combustion engine. This NSR catalyst serves to occlude or store NOx contained in an incoming exhaust gas when the oxygen concentration of the exhaust gas is high, and to reduce the occluded or stored NOx when the oxygen concentration of the incoming exhaust gas becomes low and when a reducing agent exists.
The sulfur oxides (SOx) generated by the combustion of sulfur components included in fuel are occluded or stored by this NSR catalyst, similar to NOx. The SOx stored in this manner is more difficult to be released than NOx, and is accumulated into the NSR catalyst. This is referred to as sulfur poisoning. An NOx purification (i.e., removal and/or reduction) rate in the NSR catalyst is decreased due to this sulfur poisoning, and hence, it is necessary to carry out sulfur poisoning recovery treatment at appropriate times. This sulfur poisoning recovery treatment is carried out by circulating the exhaust gas, of which the oxygen concentration has been made low, through the NSR catalyst, with the temperature of the NSR catalyst being made high.
Here, there has been known a technique in which when the sulfur poisoning of the exhaust gas purification catalyst is recovered by supplying fuel to the exhaust gas purification catalyst in an intermittent manner, the time of supplying the fuel is made longer in accordance with the increasing degree of deterioration of the exhaust gas purification catalyst (for example, refer to a first patent document).
In addition, there has been known a technique in which when the SOx adsorbed or stored in the NOx catalyst is desorbed or released, the operating state of an internal combustion engine is controlled in accordance with the amount of SOx stored in the NOx catalyst which is calculated based on the degree of deterioration of the NOx catalyst (for example, refer to a second patent document).
Moreover, there has also been known a technique in which when the sulfur poisoning of an NOx catalyst is detected which is arranged at a location downstream of a merged portion of a plurality of exhaust passages provided for each group of cylinders in an internal combustion engine, sulfur components are made to release from the NOx catalyst by individually or separately controlling the operations of the individual cylinder groups in the internal combustion engine according to the temperature of an exhaust gas purification catalyst arranged in each of the plurality of exhaust passages, respectively (for example, refer to a third patent document).
However, an NOx selective reduction catalyst (hereinafter also referred to as an SCR catalyst) can be provided at the downstream side of an NSR catalyst. This SCR catalyst is a catalyst which serves to carry out selective reduction of NOx by means of a reducing agent. An NOx purification (removal and/or reduction) rate in this SCR catalyst is affected by the influence of the state of the NSR catalyst. That is, NH3, which becomes the reducing agent for the SCR catalyst in the NSR catalyst, is generated, but the amount of NH3 thus generated changes in accordance with the state of the NSR catalyst.
Here, in this conventional technique, no mention is not made to sulfur poisoning recovery treatment of the NSR catalyst in the case of the SCR catalyst being provided at the downstream side of the NSR catalyst. For this reason, there is a fear that the sulfur poisoning recovery of the NSR catalyst may not be carried out in an appropriate manner. As a result of this, there is also a fear that the purification performance or ability of the SCR catalyst can not be exhibited.